Alterations in cancer cell metabolism are thought to represent an important Achille's heel that can be exploited for developing targeted cancer therapies. In particular, de novo lipogenesis (DNL) is increasingly recognized as an essential pathway for cancer growth due to its role in generating new cell membranes and signaling molecules. However, little is known about what regulates DNL in cancers. We propose that a greater understanding of i) the factors and mechanisms controlling DNL and ii) the nature and function of lipid products of DNL will provide new therapeutic strategies to target DNL and treat recurrent/resistant ER+ breast cancers. Using a variety of approaches and models, we have established that SREBP1 and 2, the master transcriptional regulators of fatty acid and cholesterol synthesis, respectively, regulate key enzymes for lipid synthesis, as well as DNL itself, in endocrine sensitive and resistant estrogen receptor positive breast cancer cells. Moreover, SREBP activity is essential for i) hormone-dependent proliferation of ER+ breast cancer cells that are sensitive to endocrine therapy, ii) hormone-independent proliferation of endocrine-resistant ER+ breast cancer cells, and iii) invasion of endocrine-resistant cells. We find that SREBP expression and activity are hormonally regulated by estradiol (E2) and/or the growth factor IGF-1 in several ER+ breast cancer cell lines. Additionally, we have identified two potential mechanisms by which E2 and IGF-1 may regulate SREBPs expression and activity. Based on these findings, we hypothesize that hormonal regulation of SREBP expression and activity leads to de novo synthesis of specific lipid mediators that are essential for tumor growth in endocrine-sensitive and resistant breast cancers. To test this hypothesis, we propose to conduct studies in in cell line-derived and patient-derived tumor xenografts to 1) define the function of SREBP activity in ER+ breast tumors, 2) identify biologically important lipid products of DNL, and 3) investigate hormonal regulation of DNL in growing ER+ breast tumors. Together, the results of these studies will advance our understanding of the regulation and function of DNL in breast cancer, including differences between ER+ cancers that are sensitive or resistant to endocrine-therapy. By gaining a deeper understanding of these differences, our work will provide a compelling basis for developing novel therapeutic strategies to target DNL and its lipid products in recurrent/resistant ER+ breast cancers.